Cleavage - Introduction

A 16-cell L. variegatus embryo, viewed from the side.

Sea urchins undergo radial cleavage, as do "typical" deuterstomes,
such as chordates, ascidians, and other echinoderms. Like embryonic cleavages
in other organisms, sea urchin cleavage divisions are reductive, i.e., the
cleavages result in more cells, but without an increase in the total cellular
volume of the embryo. The next few pages provide representative pictures
of cleavage stage embryos, and discuss the mechanisms and consequences of
cleavage in sea urchin embryos.